These have been attached for many years. I walk on them. They seem sturdy.
They are not held by compression because there is even a gap between the end of the stairs and the deck. They are attached to no posts along their run. As far as I can tell this run of 18 feet of stairs is only held up by maybe six nails. What am I missing?
Not that way. 
Its a good thing you have that buy back guarantee. You might need it on that one.
I have always been fond of having the initial step of the carriage be at the top of the landing. That way the plumb cut on the stringer is resting entirely on the rim joist. Rough to rough and drop the stringers (add one additional tread thickness to the rise) if you finish up on concrete.
More than likely the stringers are securely affixed at the base and are preventing them from slipping out…
You also have a skirt board on the outside which has effectively doubled the strength of the stringers as it doesn’t have the traditional cut applied at the top.
Why ?
Sounds good to me .
Try post #7: http://www.nachi.org/forum/f16/stringer-install-46307/#post600278
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